1. Technical Field of the Invention
This invention relates to a method for the manufacture and purification of methyl tertiary butyl ether. More particularly, this invention relates to a method for the manufacture of methyl tertiary butyl ether from tertiary butyl alcohol and methanol and for the purification of the methyl tertiary butyl ether formed by the reaction.
2. Prior Art
In U.S. Pat. No. 4,144,138 (1979) to Rao et al., there is disclosed a method for recovering methyl tertiary butyl ather from etherification reaction effluent by azeotropic distillation to recover methanol-ether azeotrope overhead which is water-washed to give pure ether raffinate, the latter being azeotropically distilled to yield ether-methanol overhead which is recycled to water washing.
The preparation of methyl tert-butyl ether from methyl and tert-butyl alcohols is discussed in S. V. Rozhkov et al., Prevrashch Uglevodorodov, Kislotno Osnovn. Geterogennykh Katal. Tezisy Dokl., Vses. Konf., 1977, 150 (C. A. 92:58165y). Here the TBA and methanol undergo etherification over KU-2 strongly acidic sulfopolystyrene cation-exchangers under mild conditions. This reference contains data on basic parameters of such a process.
3. Background Information
Methyl tert-butyl ether is finding increasing use as a blending component in high octane gasoline as the current gasoline additives based on lead and manganese are phased out. Currently all commercial processes for the manufacture of methyl tert-butyl ether are based upon the liquid-phase reaction of isobutylene and methanol catalyzed by a cationic ion-exchange resin.
With the expanding use of MTBE as an acceptable gasoline additive, a growing problem is the availability of raw materials. Historically, the critical raw material is isobutylene (Oil and Gas J., Jun. 8, 1987, p. 55). It would be advantageous, therefore, to have a process to make MTBE that does not require isobutylene as a building block. It would be advantageous to have an efficient process for making MTBE by reaction of methanol with tertiary butyl alcohol, since t-butanol (TBA) is readily available commercially through isobutane oxidation.
It is known to react methanol with tertiary butyl alcohol in the presence of a catalyst in order to produce methyl tertiary butyl ether. A wide variety of catalysts have been suggested for this purpose.
In U.S. Pat. No. 2,282,469 to Frolich there is disclosed a process for preparing methyl tertiary butyl ether over a catalyst comprising Kieselguhr impregnated with phosphoric acid at a temperature of about 175.degree. F. to 350.degree. F.
Japanese Patent No. 0007432 teaches the use of zeolites to make dialkyl ethers containing primary or secondary alkyl groups. The zeolites have a porous structure and are represented by: EQU M.sub.2/n O.multidot.Al.sub.2 O.sub.3 .multidot.xSiO.sub.2 .multidot..sub.y H.sub.2 O
where M is an alkali metal or alkaline earth metal cation or organic base cation, n is the valence of the cation and x and y are variables.
U.S. Pat. No. 4,058,576 to Chang et al. teaches the use of (pentasil-type) aluminosilicate zeolites, such as ZSM-5, having a pore size greater than 5 angstrom units and a silica-to-alumina ratio of at least 12, to convert lower alcohols to a mixture of ethers and olefins.
In U.S. Pat. No. 4,822,921 there is disclosed a method for producing MTBE by reacting tertiary butyl alcohol and methanol in the presence of a catalyst comprising an inert support, such as titania, having a phosphoric acid impregnated thereon.
U.S. Pat. No. 4,827,048 discloses a method for producing MTBE by reacting tertiary butyl alcohol and methanol in the presence of a catalyst comprising a heteropoly acid such as 12-tungstophosphoric acid or 12-molybdophosphoric acid on an inert support, such as titania.
Two of the principal by-products formed during the reaction of the methanol with the tertiary butyl alcohol are water and isobutylene. Methanol and methyl tertiary butyl ether form an azeotrope which is broken only with difficulty and therefore the separation of methanol from MTBE during the recovery of purified methyl tertiary butyl ether presents a serious problem.
In U.S. Pat. No. 4,820,877, separation of methanol from MTBE is accomplished by using a refinery fuel gas to enhance the separation of methanol into the overhead stream of a distillation column.
In U.S. Pat. No. 4,814,517, separation of methanol from MTBE is accomplished by using a silica gel to preferentially adsorb methanol from an MTBE stream and by periodically regenerating the silica gel.
In U.S. Pat. No. 4,798,674, separation of methanol from MTBE is accomplished by using a membrane of cross-linked polyvinyl alcohol or a quaternary ammonium ion resin. Methanol preferentially permeates through the membrane increasing the MTBE concentration of the charge liquid.
In U.S. Pat. No. 4,759,850, separation of methanol from MTBE is accomplished by reverse osmosis.
In U.S. Pat. No. 4,440,963, separation of methanol from MTBE is accomplished by adding an agent such as 2-methyl pentane or Freon 113 to form an azeotrope with methanol. This azeotrope is recovered overhead giving a methanol-free MTBE bottoms product.
As recognized by Rao et al. in U.S. Pat. No. 4,144,138, isobutylene is formed as a by-product when methanol is reacted with tertiary butyl alcohol. In accordance with the Rao process, the isobutane is separated from the reaction product in an initial azeotropic distillation step as a noncondensable gas. Rao teach that the part of the isobutane may be flashed from the reaction product for recycle, depending upon purity.